While McCormick is slightly behind Oz after the May 17 vote, he is well ahead of his opponent in absentee ballots, according to polling firm Edison Research. McCormick has received 45,794 mail-in votes, compared with Oz, who has 32,944.
In a statement, McCormick's campaign said it sued on Monday in Pennsylvania's Commonwealth Court "to compel the counties to follow the [Republican-leaning] Third Circuit Court order from last week stating that undated ballots returned on time be counted."
The statement added that the ballots "are postmarked upon arrival to county boards of elections and, therefore, already dated and proven to be timely."
It was not clear how many mail-in ballots lack a handwritten date, and whether counting them could help McCormick or make a recount less likely. State election officials expect to know this week whether a recount will be needed.
Pennsylvania's Department of State, which oversees elections, said it agreed that undated ballots must be counted in the May 17 race, but advised they be "segregated" and "appropriately logged pending litigation."
"A determination on whether the segregated tabulations will be used in certifying elections has not yet been made, given the ongoing litigation," it said.
The Pennsylvania Republican Party said on Twitter that they "absolutely object to the counting of mail-in ballots. Pennsylvania law and our courts have been very clear that undated ballots are not to be counted."
In a statement on Twitter, Oz called McCormick's lawsuit "a tactic that could have long-term harmful consequences for elections in the Commonwealth of Pennsylvania."
(Reporting by Rami Ayyub; editing by Ross Colvin and Jonathan Oatis)
